C/C composites are resistant to heat and chemicals and lightweight, and have a high strength. Accordingly, C/C composites are useful as heat-resistant materials used in non-oxidizing atmosphere, and are particularly used for brake discs and pads or the like of airplanes and automobiles because of their superior resistance to heat attack (for example, Japanese Unexamined Patent Application Publication No. 7-33543).
Unfortunately, carbon materials are generally oxidized at about 500° C. or more, and are therefore not used in high-temperature air except for being used in a very short time.
In order to prevent the physical or chemical degradation of the C/C composite resulting from oxidation, or, if it is used as a sliding member, in order to prevent the reduction of mechanical strength resulting from oxidation and abrasion, the reduction of frictional properties at low temperatures, and the reduction of frictional properties caused by rain drops and other water attached to the sliding member, the C/C composite is impregnated with melted metal Si to react with the Si and thus to transform the carbon of the C/C composite into SiC. In general, PAN (polyacrylonitrile)-based carbon fiber is used as the carbon fiber of the C/C composite that is to be impregnated with melted metal Si to form SiC.
In Japanese Unexamined Patent Application Publication No. 2000-351672, a C/C composite is produced by a method including a step for preparing a material comprising a bundle of carbon fiber wherein a powdery binder pitch acting as a matrix in the material comprising the carbon fiber bundle and ultimately acting as free carbon being free from the carbon fiber bundle is added to the carbon fibers aligned in a single direction, and then phenol resin powder or the like is added thereto. The carbon fibers thus prepared are covered with a flexible coating made of a resin such as a thermoplastic resin to produce a preformed yarn used as a flexible intermediate material. The preformed yarn is formed into sheets by a method disclosed in Japanese Unexamined Patent Application Publication No. 2-80639. The C/C composite is produced by stacking a desired number of the sheets one on top of another in such a manner that the directions of the carbon fiber orientations are perpendicular to each other, and subsequently performing predetermined process steps. The resulting C/C composite is impregnated with metal silicon, and thus a SiC—C/C composite material having a porosity of 5% is produced.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-351672
Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-33543
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2-80639
Since the SiC—C/C composite material of Patent Document 1 is a stack of carbon fiber sheets whose fiber orientations are perpendicular to each other, it is considered that the composite material has a high tensile strength in the longitudinal direction of the fibers (in the 0° direction or the 90° direction), but a low tensile strength in directions of 45° with respect to the perpendicular carbon fibers.
In addition, the inventors of the present invention have found that in a SiC—C/C composite prepared by impregnating a C/C composite using PAN-based carbon fibers with a melted metal Si for transformation into SiC, the reinforcing effect of the carbon fibers is degraded, so that the SiC—C/C composite has no more than a tensile strength close to that of carbon materials not reinforced with carbon fibers. For example, Comparative Examples 1 and 2, described later, cannot achieve a tensile strength of more than 30 MPa in a specific tensile test.